. >> reporter: wolf, the challenge is for the obama administration to figure out exactly what bashar al assad is up to with his deadly arsenal. the horror remains unspeakable. 25 years ago saddam hussein unleashed one of the worst poison gas attacks in history. thousands were killed. now, in syria, u.s. concerns are growing by the hour that bashar al assad may be planning the same thing against his citizens. >> the intelligence that we have raises serious concerns that this is being considered. >> reporter: u.s. intelligence shows syria has mixed chemical compounds needed to make a deadly agent that can quickly kill thousands. >> the united states and our allies are facing the prospect of an imminent use of weapons of mass destruction in syria. and this may be the last warning we get. >> reporter: the u.s. is not precisely saying what the syrians are doing, but there are two ways of mixing elements to make a saren filled weapon. >> you have to handle it very, very carefully. a drop will kill you. often it's done at the last minute. there's another style. >> reporter: chemicals placed